The polymerization of ethylene unsaturated monomer compounds, e.g., the polymerization of styrene, acrylic acid or methacrylic acid or their ester derivatives as well as the hardening of polyfunctional acrylic acid esters or methacrylic acid esters, are initiated by free radicals. These free radicals can be formed, among other methods, by means of redox systems based on organic peroxide compounds. Aromatic tertiary amines have proven themselves as suitable accelerators for this redox polymerization. In particular, dialkylated anilines are used as such amine accelerators in polymerization by means of peroxides. However, in the production of polymer materials based on polyfunctional acrylic acid esters, which are used, among other things, in dentistry as tooth filling materials or for purposes of dental prosthesis, it has been shown that especially the formerly much-used diethyl aniline, as well as hydroxyalkylated aniline derivatives, leads after a time, often only one or two years, to a discoloration of the polymers. This is especially troubling in the use of these substances for dental fillings in the front of the mouth. From U.S. Pat. No. 3,541,068 it is known that N,N-bis-(hydroxyalkyl)-3,5-xylidines are especially suitable accelerators for the polymerization of acrylic or methacrylic esters of aromatic or aliphatic di- or trioles. But after a time, the products made with these special amines also show discoloration.
Because of the great variety of substances which are used in human nutrition, the precise mechanism for the discoloration of polymers in dental fillings produced from unsaturated monomers by means of an amine-activated redox polymerization presents a very difficult problem to solve which is all the more difficult as the accompanying processes and chemical changes of the material have not yet been fully explained. Since these accelerators exhibit basic characteristics, acidic, colored food substances can enter into saline or complex unions with the amines which have remained in the polymeric material. There is also the possibility that these amines themselves may change in the course of time through oxidative processes and that coloring compounds may results. Because of these problems, there is a disinclination to use these amine accelerators at all in redox systems to produce polymers in which discoloration is disadvantageous.